In recent years, for example, various techniques for purifying exhaust gas of a diesel, engine have been proposed due to the increasing of environment problems. The approach in this type of exhaust gas purification technique is performed, for example, from the viewpoint of improvement in the engine combustion, improvement in the post-processing technique, or improvement in fuel.
Among these viewpoints, as for the improvement in the engine combustion, the improvement in a fuel injection system is regarded as the most important factor, and for example, there has been proposed a technique for realizing and improving a high-pressure injection and a multi-stage injection using a common rail system which is mainly used as the fuel injection system of a diesel automobile (for example, see Non Patent Literature 1).
Further, as for the improvement in the fuel, a technique using emulsion fuel has been developed, and some positive results have been obtained. Furthermore, as a method of reducing nitrogen oxide (NOx) contained in the exhaust gas of a diesel engine, there have been known effective methods such as a method of mixing water into fuel, a method of spraying water to a combustion chamber in addition to fuel, and the like. These methods use the principles in which NOx is reduced with a decrease in the combustion temperature through the mixing of water into fuel and the combustion efficiency is improved by oxygen contained in steam.